C++11 @ Qt 5.0. ○ constexpr added to many types. ○ move semantics added to
a few types. ○ initializer_list added to most types. ○ very few N-ary ctors ...
Marc Mutz KDAB (DE), Qt Widget Maintenance
Speed Up Your Qt 5 Programs Using C++11
Overview
C++11 @ Qt 5.0
●
constexpr added to many types
●
move semantics added to a few types
●
initializer_list added to most types
●
very few N-ary ctors marked explicit, N ≥ 2
●
= delete used almost ubiquitously
●
noexcept added in a few central places
●
other features not very usable in APIs
A Simple Example
Prevents optimisations...
Results
Less!
Less!
The C++11 Free Lunch Less! (GCC 4.8-20120823 / AMD64 / Linux) Baseline (int main() {})
Whodunnit?
The Qt-Project did...
Whodunnit? This is magic...
Whodunnit?
C++98
Whodunnit?
C++11
constexpr?
constexpr
●
New keyword in C++11
●
Can be applied to
●
●
(Free and Member) Functions
●
Variables
●
Constructors
Enables Evaluation at Compile-Time
constexpr
● ●
New keyword in C++11
Can be appliedconstexpr to ● ● ●
●
constructor
(Free and Member) Functions (also requires a trivial destructor) Variables Constructors
“Literal Type”
Enables Evaluation at Compile-Time
const Type == compile-time constant
constexpr
constexpr
.section .rodata “read-only data” .align 16 .type _ZL8triangle, @object .size _ZL8triangle, 24 _ZL8triangle: .long -50 .long 0 .long 50 .long 0 .long 0 .long 100
C++11
constexpr
And that is new... ...how? Well...
constexpr
“read-only data” [...] .section .ctors,"aw",@progbits .align 8 .quad _GLOBAL__sub_I_main .local _ZL8triangle .comm _ZL8triangle,24,16
C++98
constexpr
.type _GLOBAL__sub_I_main, @function _GLOBAL__sub_I_main: movl $-50, _ZL8triangle(%rip) “D movl $0, y4+_ZL8triangle(%rip) nam ic I movl $50, 8+_ZL8triangle(%rip) n it iali movl $0, 12+_ZL8triangle(%rip) sat io n movl $0, 16+_ZL8triangle(%rip) ” movl $100, 20+_ZL8triangle(%rip) ret “read-only data” [...] .section .ctors,"aw",@progbits .align 8 .quad _GLOBAL__sub_I_main .local _ZL8triangle .comm _ZL8triangle,24,16
C++98
constexpr
Co sts ?
.type _GLOBAL__sub_I_main, @function _GLOBAL__sub_I_main: movl $-50, _ZL8triangle(%rip) “D movl $0, y4+_ZL8triangle(%rip) nam ic I movl $50, 8+_ZL8triangle(%rip) n it ? my> iali n movl $0, 12+_ZL8triangle(%rip) o i t a s anat o sat s i l a i t io n i movl $0, 16+_ZL8triangle(%rip) rey' n I g / < ” on e c movl $100, 20+_ZL8triangle(%rip) Ra a t a :D g n ret i d Or a e r h “read-only data” de ltit [...] u M ro .section .ctors,"aw",@progbits fI ni t ia .align 8 lis .quad _GLOBAL__sub_I_main at io n? .local _ZL8triangle .comm _ZL8triangle,24,16 art up
? ? ?
St
y l s u
S
o i r e
C++98
constexpr
constexpr
_Z3onev: subq movq movl movl movq movq call addq ret
$40, %rsp %rsp, %rdi $-1, (%rsp) $-1, 4(%rsp) $0, 8(%rsp) $0, 16(%rsp) _Z10checkIndexRK11QModelIndex@PLT $40, %rsp _Z3twov: leaq jmp
_ZL4root(%rip), %rdi _Z10checkIndexRK11QModelIndex@PLT
Tail Call Optimisation
constexpr
_Z3onev: subq $40, %rsp movq %rsp, %rdi ● compilers don't fold instances of movl $-1, (%rsp) movl $-1, 4(%rsp) ● standard doesn't permit it movq $0, 8(%rsp) movq $0, 16(%rsp) ● addresses call _Z10checkIndexRK11QModelIndex@PLT must be unique addq $40, %rsp ● ret
literal types
you need to do the folding yourself
●
learn to love
_Z3twov: _ZL4root(%rip), %rdi staticleaq const variables :) jmp _Z10checkIndexRK11QModelIndex@PLT
Tail Call Optimisation
virtual functions
Talking about embarrssments...
Error: `timerEvnet(QTimerEvent*)` doesn't override anything!
. . . e m i t n u r ) : p t u n e d e m e p p o l s e t ' v n e s d e t o u D ...b
Another Simple Example
C++98: QString::fromUtf8() C++11: copy of a pointer
C++11: won't throw
Guidelines
●
add Q_DECL_OVERRIDE to virtual overrides
●
use static const type in favour of temporaries ●
in apps, not DLLs/SOs/DYLIBs
●
even when just default-constructed
●
prefer unnamed over named temporaries
●
use QStringLiteral
●
qDebug() doesn't throw anymore
C++11 @ Qt 5.1 (planned)
●
add more constexpr
●
add more noexcept
●
add all missing move ctors
●
experiment with rvalue refs on *this
●
experiment with extern templates
●
mark N-arg ctors explicit, N ≥ 2
●
add Q_DECL_OVERRIDE everywhere
●
implement C++11 API on our containers
Q&S
Questions? Suggestions?
